Start-up circuit for providing a start-up voltage to an application circuit

ABSTRACT

A startup circuit for providing a startup voltage from a high voltage DC bus voltage to an application circuit including an integrated circuit package for at least a control circuit for driving at least one power switch of the application circuit; a dropping resistor in the integrated circuit package having a first terminal for coupling to the high voltage DC bus and for dropping the high voltage DC bus voltage to a reduced voltage level at a second terminal; and a low voltage regulator in the package coupled to the second terminal for providing a startup regulated low voltage DC output at a preset level for powering at least one part of the application circuit during startup of the application circuit. The application circuit may be, for example, a switching mode power supply.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit and priority of U.S.Provisional Application No. 60/600,046 filed Aug. 9, 2004 and entitledSTART-UP RESISTOR FOR HIGH VOLTAGE SWITCHING REGULATOR, the entiredisclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a start-up circuit for providing astart-up voltage from a high voltage DC bus to a high voltageapplication circuit, for example, wherein the application circuit maycomprise a switching regulator circuit coupled to a high voltage DC bus.

In the past, start-up circuits comprising external resistors areprovided to application circuits wherein the application circuitcomprises an integrated circuit. It is desirable to be able to eliminatethe external resistor. However, this has been difficult to achieve inthe past due to the high voltage. Further, it is also desirable toprovide a start-up circuit which has an automatic shutoff feature whichshuts down the start-up circuit when the voltage regulator output issensed, i.e., when the voltage regulator output is generated by thevoltage regulator circuit. In this way, maximum power dissipation isconfined to a small time interval during start-up.

Further, it is desirable to be able to provide overcurrent protectionthat limits power dissipation in the start-up circuit in the event of anoutput short or overload condition.

As described previously, in the past, such start-up supplies wereprovided through an external resistor which increases cost and furtherwhich does not have the automatic shutoff capability nor overcurrentprotection.

The main purpose of this circuit is to supply a switching voltageregulator with power upon start-up from a high voltage DC bus before theregulator can self-supply. A typical DC bus voltage may be approximately600 volts, for example.

SUMMARY OF THE INVENTION

The invention provides a number of advantages. A dropping resistor isincorporated in the integrated circuit package for the controller of theapplication circuit, for example a switching regulator or the controland drive circuit of a switch regulator output circuit.

In addition, an automatic shutoff feature which shuts down the circuitwhen the voltage regulator output is provided. In this way, maximumpower dissipation is confined to a small time interval during start-up.

Furthermore, an overcurrent protection circuit that limits powerdissipation in the circuit in an output short or overload condition isprovided.

According to the invention, the above objects are achieved by a startupcircuit for providing a startup voltage from a high voltage DC busvoltage to an application circuit comprising an integrated circuitpackage for at least a control circuit for driving at least one powerswitch of the application circuit; a dropping resistor in the integratedcircuit package having a first terminal for coupling to the high voltageDC bus and for dropping the high voltage DC bus voltage to a reducedvoltage level at a second terminal; further comprising a low voltageregulator in the package coupled to the second terminal for providing astartup regulated low voltage DC output at a preset level for poweringat least one part of the application circuit during startup of theapplication circuit.

BRIEF DESCRIPTION OF THE DRAWING(S)

The invention will now be described in greater detail in the followingdetailed description with reference to the drawings in which FIG. 1shows the start-up circuit according to the present invention.

Other objects, features and advantages of the present invention will beapparent from the detailed description which follows.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Turning now to the drawings, FIG. 1 shows the start-up circuit accordingto the present invention. The start-up circuit comprises,illustratively, an NMOS start-up circuit showing the high and lowvoltage parts of the circuit. The high voltage section is containedwithin a high voltage well HW. The border of the high voltage well HWhas a polysilicon resistor (spiral R) which may be a spiral shapedpolyresistor which is used to shape the electric field. One terminal ofthe polyresistance is in the high voltage well and is connected to thehigh voltage DC bus VB. The other terminal is in the low voltage sideand is connected to the gate of the high voltage NMOS Q1. The value ofthe resistor spiral R is in the tens of megohms. It serves to turn onthe high voltage transistor Q1.

As shown in FIG. 1, the application circuit 10 is provided with voltagefrom the high voltage bus. The application circuit may be a switchingmode power supply or the control and drive portion for the output stage12 of a switching mode power supply (SMPS), shown illustratively as asynchronous buck converter output stage, although the SMPS can be anyother type of SMPS, such as a boost converter, back-boost, flybackforward converter, etc. According to the invention, the applicationcircuit 10 is typically contained within the square package (IC package)as the start-up circuit. Before the application circuit, which maycomprise a switching regulator, can self regulate to apply the outputvoltage V0, it is provided with the start-up voltage VCC by the start-upcircuit shown. Once the application circuit begins to supply theregulated output voltage, the high voltage start-up circuit is shut off.

The start-up circuit shown can also be used as a very low current powersupply X for circuits such as comparators, for example, drawing smallcurrents, for example, tens of microamps at about 12 volts. The VCCpoint is maintained at about 12 volts in the illustrated embodiment bythe current flowing in the branch with the zener diode, shownillustratively as two zener diodes Z1 and Z2. In the illustratedembodiment each zener diode has an avalanche voltage of 5 volts for atotal of 10 volts avalanche voltage. The voltage drop across thesezeners plus the voltage drop across resistor R3 add to make VCC. Thisgives about 10 volts plus the threshold of Q2 (about 1.5 volts) for VCC.

This start-up circuit regulates VCC through a feedback mechanism asfollows. If the voltage at VCC increases for some reason above itssteady state value, this causes the voltage across R3 to increasecausing Q2 to turn on more heavily, which then causes the gate nodevoltage of Q1 to decrease. This decrease in gate voltage decreases thedrain current of Q1 which then closes the feedback loop by causing adecrease in the voltage of VCC which initially increased. The oppositesituation results if the change in VCC is opposite, i.e., if VCCdecreases.

In the desired operation, the output is taken from the point labeledVCC. As mentioned above, the output voltage is about 12 volts in theillustrated embodiment. This voltage is only needed momentarily whilethe application circuit is starting up. Once the application starts upit is desirable to turn off the start-up circuit to save power. Aftersupplying and powering up the application circuit, for example, aswitched mode power supply, Q2 senses the power supply output at VCC andprovides the shutoff function of the circuit by sensing the voltageacross R3. The higher voltage at VCC and hence across R3 will turn on Q2more (V_(T) approximately 1.5 volts) and will decrease the gate drive ofthe supplying transistor Q1. After the shutoff, the symmetrical deviceQ3 has its drain connected to ground and its gate/source to VCC. Thiscauses Q3 to turn on with the source and drain interchanged. Then, R1 inthe interchanged source circuit of Q3 limits the power dissipationduring the shutoff operation.

The circuit also provides short circuit protection. If VCC were to beshorted or overloaded, all that portion of the circuit below the VCCpoint would be eliminated and the source of Q1 would be connected toground through resistor R2, which may be, for example, 20 K ohms. Thiswill cause too much power dissipation from VB without a current limit.In this case, Q3 senses a greater voltage drop across R2 in the shortcircuit operation and turns on, bringing down the gate voltage of Q1,limiting the maximum current.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art.Therefore, the present invention should be limited not by the specificdisclosure herein, but only by the appended claims.

1. A startup circuit for providing a startup voltage from a high voltage DC bus voltage to an application circuit, the startup circuit comprising: an integrated circuit package for at least a control circuit for driving at least one power switch of the application circuit having a low voltage terminal; a dropping resistor in the integrated circuit package having a first terminal for coupling to the high voltage DC bus and a second terminal, the dropping resistor dropping the high voltage DC bus voltage to a reduced voltage and providing the reduced voltage at the second terminal; a low voltage regulator coupled to the second terminal for using the reduced voltage for enabling generation of a regulated startup low voltage DC output at a preset level at the low voltage terminal for powering at least one part of the application circuit during startup of the application circuit, wherein the high voltage DC bus voltage is the only voltage source provided externally to the integrated circuit package and wherein the low voltage regulator comprises a first high voltage switch in series circuit with a first resistance, the series circuit coupled between said first terminal and said low voltage terminal; a second switch coupled between said second terminal and a terminal having a reference potential and having a control electrode coupled to a voltage sensor sensing said low voltage DC output; a voltage reference source for setting the low voltage DC output; said voltage reference source and voltage sensor being coupled in series between said low voltage DC output and said terminal having said reference potential; whereby when said low voltage DC output at the low voltage terminal changes from the preset level, conduction through said second switch changes, thereby changing the voltage at the control electrode of the first high voltage switch to drive the low voltage DC output back to the preset level.
 2. The startup circuit of claim 1, wherein if the low voltage DC output increases from the preset level, a voltage across said voltage sensor increases, causing said second switch to increase its conduction of current, thereby reducing the voltage at the control electrode of said first high voltage switch, thereby decreasing the current in said first high voltage switch and reducing the low voltage DC output to the preset level.
 3. The startup circuit of claim 2, wherein if the low voltage DC output decreases from the preset level, a voltage across said voltage sensor decreases, causing said second switch to decrease its conduction of current, thereby increasing the voltage at the control electrode of said first high voltage switch, thereby increasing the current in said first high voltage switch and increasing the low voltage DC output to the preset level.
 4. The startup circuit of claim 1, wherein said second switch senses said low voltage DC output via said voltage sensor, so that, when said low voltage DC output is present after startup is higher than the preset level, said second switch will increase its conduction of current to decrease the voltage at the control electrode of the first high voltage switch such that the first high voltage switch turns off.
 5. The startup circuit of claim 4 further comprising a third switch in series with a series resistance coupled between said second terminal and said low voltage terminal for limiting power dissipation in said second switch when said first high voltage switch is turned off.
 6. The startup circuit of claim 5, wherein said third switch has a control electrode coupled to said low voltage terminal through said first resistance whereby, when said second switch turns on when the output of the switching regulator circuit is present after startup on the low voltage DC output, said third switch turns on so that current flows through said series resistance.
 7. The startup circuit of claim 6, wherein said third switch comprises a symmetrical MOSFET, whereby, when said third switch turns on, the source and drain of the third switch interchange.
 8. The startup circuit of claim 5, wherein said third switch senses a voltage across said first resistance, whereby, if said voltage across said first resistance indicates a short circuit condition at said low voltage terminal, said third switch operates to limit the current in said first high voltage switch to a preset safe level.
 9. The startup circuit of claim 8, wherein, if the voltage across said first resistance indicates a short circuit condition at said low voltage terminal, said third switch turns on, reducing the voltage at said control electrode of said first switch, thereby limiting the current in said first high voltage switch to a preset safe level.
 10. The startup circuit of claim 1, wherein the voltage reference source comprises at least one Zener diode.
 11. The startup circuit of claim 5, wherein said first high voltage, second and third switches comprise MOFSETs.
 12. The startup circuit of claim 1, further comprising a transient protection circuit coupled to the control electrode of said first high voltage switch. 